Modulating system



y 1939- E. B. FERRELL 2,159,020

' AMODULATING SYSTEM Filed June 9, 1934 s Sheets-Sheet 1 F/ g a E g V aLon) 2 msauzncr AMP.

man T FREQUENCY AMP.

lNVENTOR E B. F E RRE L L erg/aw ATTORNEY y 9 E. B. FERRELL 2,159,020

MODULATING SYSTEM Filed June 9, 1954 s Sheets-Sheet 2 FIG. 4

INVENTOR E. 8 FERRELL BYZW A TTORNE) y 1939- E. s. FERRELL 2,159,020

MODULATING SYSTEM Filed June 9, 1934 2' Sheets-Sheet a FIG. 7

lNl EN m E. 8. FERRELL ATTORNEY Patented May 23, 1939 PATENT OFFICEMODULATING SYSTEM Enoch B. Ferrell, Oakhurst,

Bell Telephone Laboratories,

N. J., assignor to Incorporated,

New York, N. Y., a corporation of New York Application June 9, 1934,Serial No. 729,735 2 Claims. (01. 179-111) This invention relates tomodulating systems and more particularly to means for regulating thetransmission characteristics for such systems.

I The invention has as its main object to reduce wave distortion in amodulator, particularly envelope distortion of a modulated wave.

Another object is to remove from an alternating current or carrier waveany undesired modu- 10 lation such as noise effects and slow amplitudevariations.

' Noise and modulation effects have been successfully controlled inamplifiers by means of negative feedback or degeneration. The underlyingprinciples involved in this method of control are disclosed by H. S.Black in an article entitled Stabilized feedback amplifiers, publishedin the January 1934 issue of the Bell System Technical J ournal, onpages 1-18 and in a copending appli- 20- cation, Serial No. 606,871,filed April 22, 1932, and

assigned to the same assignee as the present application. In amodulator, however, the problem of noise and distortion reduction issomewhat complicated by the necessary frequency changes 5 which occur.

In accordance with the invention, a detector is employed to derive adetected signal from a modulated carrier wave. In the absence offeedback in the system the detected signal varies somewhat 30- from theoriginal signal in its wave form according to the amount of noise anddistortion introduced by the modulator. To reduce this distortion thedetected signal is fed back degeneratively upon the signal input of themodulator.

35' By making the fed-back component large compared with the originalsignal input and controlling its amplitude, the envelope distortion ofthe modulated wave may be reduced as much as may be desired.

40 Preferably a rectifier having a linear characteristic is used as themeans for detecting the signal from the modulated wave since such arectifier producesa faithful copy of the envelope of the modulated waveand introduces no addi- 45 tional signal distortion by itself.

In a modified form of the invention the principle of degenerativefeedback is used to remove noise components and other undesiredmodulations from a carrier wave, thus providing a pure '50 sinusoidalwave of constant amplitude which may be supplied -to a modulatingamplifier for'signal modulation. V

The invention will be more fully understood from the following detaileddescription and from 55 the accompanying drawings of which:

Fig. 1 shows a radio transmitting system embodying this invention;

Fig. 2 shows an application to the Heising system of modulation;

Fig. 3 shows the application to a grid circuit 5' modulator;

Fig. 4 shows the elimination of a separate detector at the transmittingstation;

' Fig. 5 shows a reversed feedback arrangement I adapted to a screengrid amplifier;

Fig. 6 shows a modification of the system of Fig. 2 in order to utilizea signal input amplifier of the push-pull type; and

Fig. 7 shows a transmitting system incorporating initial noise reductionin a carrier supply amplifier.

In Fig. 1, a balanced high frequency generating system I is shownconnected to a transmitting antenna system 2 through a high frequencyam- I plifier 3. A signal source 4 is associated with the output circuitof the generating system I by means of a transformer 5, a bridge network6 and a low frequency amplifier 1. A receiving antenna 8 is providedwith a rectifying detector 9, H the output of the detector beingconnected through a filter I0 and transformer II to the bridge 6.Transformers 5 and I l are placed in the opposite diagonals of thebridge, with the amplifier 1 across one of the ratio arms. Dete'cv tor 9may be a diode vacuum tube or a copper oxide rectifier or otherrectifying device exhibiting a linear relationship between its inputvoltage and output current.

In the operation of the system shown in Fig. 1, the high frequencyoutput of the generator I is modulated by amplified signals from source4 transmitted through the transformer 5, the bridge 6 and theamplifier 1. The modulated high frequency wave is amplified in theamplifier 3 and radiated from the antenna 2. The antenna 8 picks upenergy from the radiated wave for detector 9, which feeds the detectedwave back in reversed phase upon the input circuit of the low frequencyamplifier 1. The feedback operates to diminish noise effects anddistortion 4 in the output wave from the system in accordance with theprinciples described in the hereinbefore mentioned paper by H. S. Black.The system diminishes noise and distortion originating in any portion ofthe loop comprising amplifier 1, generator I, amplifier 3, detector 9,and their coupling arrangements. The greatest reduction of disturbingeffects Will occur when the feedback is as strong as possible and has aphase angle close to degrees. Strong reversed 5 feedback results, ofcourse, in reduced output, but the modulation can be restored to itsformer strength by increasing the signal input. The phase requirementcan generally be met by r..- versing one winding of the transformer ll,because with high grade amplifiers the phase shift in any stage will beeither zero or degrees. If the total phase shift around the loop is 180degrees or an odd multiple thereof, the transformer may not be needed.

The general principles of reversed feedback operation are set forth inBlacks article above cited. In the application of the principles tospecific circuits it is well to note two definite limitations. One isthat the system will not compensate for distortion which comes frominterruption of the space current of a vacuum tube. Neither will itcompensate for distortion due to exceeding the saturation voltage of atube. The other limitation is imposed by the tendency to sing or set upself-sustaining oscillations. Nevertheless I have found by experimentthat a substantial reduction of noise and distortion can be secured inradio transmitters by the use of my invention.

The bridge network 6 serves to isolate the feedback circuit from thesignal supply circuit, making these circuits conjugate to each other andcapable of independent adjustment. The filter I0 is employed to preventhigh frequency feedback.

Fig. 2 shows a transmitter using the system for modulation disclosed inU. S. Patent 1,442,147 issued January 16, 1923 to R. A. Heising. Acarrier generator I! is coupled to the transmitting antenna 2 by meansof an amplifier I3. A low frequency amplifier I4 has its plate circuitcoupled to the plate circuit of the modulating amplifier I3 through achoke coil I5 as is conventional in the Heising system. The detector 9is coupled to the output circuit of amplifier I3 by high frequencycoupling I6 and to the input of amplifier I4 by the low frequencytransformer I I. The signal source 4 is associated with the input ofamplifier I4 by means of the transformer 5.

In the operation of the system of Fig. 2, the signal wave from thesource 4 is amplified in the vacuum tube I4. the amplified wave beingused to effect modulation in the tube I3. The modulated wave isimpressed upon the antenna 2 and also through coupling I6 upon thedetector 9. The detected wave is fed back upon tube MI in reversed phaseby means of the transformer II. The feedback in this case operates toreduce noise and distortion originating in the audio system and in theplate circuit of tube I3.

Fig. 3 shows, the application of the invention to a grid circuitmodulator. The carrier generator I2 is connected to the antenna 2through the modulating amplifier I3 and a power amplifier IT. The signalsource 4 is connected into the grid circuit of amplifier I3 by means ofthe transformer 5. The detector Sis coupled by high frequency couplingI6 to the output circuit of amplifier I1 and by low frequencytransformer II to the input of amplifier I3.

In the operation of the system of Fig.. 3, the

signal. wave is impressed upon the grid of tube I3. along with thecarrier wave. The. modulated output of tube I3 is amplified by tube I1and then radiated by antenna 2. The detector 9 picks up. energy from theoutput of amplifier I1 by means of the coupling I6. The detected waveis, fed back upon the grid of tube I3 in reversed phase relation to thesignal wave by means of the transformer II.

Fig. 4 shows the application of the invention to a modulating amplifierof class B or class C, that is, to an amplifier operated with suifioientnegative grid bias to cut off or nearly cut off the plate current in theabsence of impressed alter nating current excitation. In the figure thecarrier generator I2 is connected 'to 'theantenna 2 through themodulating amplifier I3 as in Fig. 2.

The signal source 4 is coupled to the input circuit of the amplifier I3by means of the bridge 6 and the transformer 5. In the opposite bridgediagonal to the transformer 5 is inserted a low frequency coil I8 whichis part of the plate circuit of the tube I3. The grid circuit of thetube I3 includes one of the ratio arms of the bridge. The plate supplybattery is shown at I9.

In the operation of the system of Fig. 4, the carrier wave is impressedupon the grid circuit of the tube I3 along with the signal wave from;the bridge 6.

tential drop in the ratio arm included in the grid circuit. Due to thestrong bias the plate current flows in pulses when the carrier andsignal waves are impressed. In the presence of 1 the carrier withoutsignals the pulses are uniform, but when the signals are applied thepulses vary in strength according to the signals. Consequently theplate' current contains a component of low frequency, whichissubstantially a faithful copy of the envelope'of the modulated wave'ina properly adjusted modulator. The low frequency wave traverses coil I8,thus feeding back upon the-grid of tube I3. This feedback is inherentlya reversed one as it will be evident from the figure that an increase.of plate current through coil I8 makes the grid more negative, while adecrease of plate current makes the grid less negative, these variationsin the grid voltage being opposite in phase a large biasing battery 2|to effect class B or 0 operation. The signal source 4 is associated withthe screen electrode of tube Zllby means of the transformer 5. Couplingbetween the plate and screen circuits at low frequencies is provided bythe transformer II.

In the operation of the system of Fig. 5, the carrier is impressed uponthe-grid and the signal.

upon the screen of the tube 20. --The modulated wave generated in theplatecircuit is radiated by the antenna 2. The low frequency componentof the plate current generated by class B or Coperation is fed backuponthe screen by means of the transformer II which is-so poled that anincrease of plate current results in a decrease in the posi-.

tive bias upon the screen and a decrease of plate current results in anincrease of .screen; bias,

which conditions evidently constitute :reversedfeedback.v

v Fig. 6 shows the use of the invention in connection with a classBsignal; amplifier in a, Heising The plate current from the battery !9passes through the bridge 6 providing a strong negative grid bias forthe tube, fixed by the po- Cit contant current modulating system. Thecarrier portion of the system is the same as shown in Fig. 2. The signalsource 4 is connected through a pair of bridge networks 25 and 26 topush-pull connected tubes 23 and 24, the grids of which are normallybiased to about the plate current cut-off point. The plate circuits ofthe tubes are coupled to the plate of tube I3 by means of a balancedtransformer 21. Two detectors 9 and 29 are connected in series aidingrelation to each other. The detectors are coupled to the antenna circuitby a high frequency coupling I6 and to the grids of the tubes 23 and 24through the bridges 25 and 26, the latter having a common point atground 80.

In the operation of the system of Fig. 6, the carrier is impressed uponthe grid of the tube I3 and the signal upon the plate as is usual in theHeising system. The modulated wave is radiated by the antenna 2 and alsoa portion of its energy is transmitted to the detectors 9 and 29 by thecoupling I6. The detected wave is fed back upon the grids of the tubes23 and 24 through bridges 25 and 26 respectively, the detector circuitbeing balanced with respect to the ground because of the symmetry of thefeedback path. While the symmetry is promoted by the use of twodetectors as shown, in some cases sufficient balance may be obtained ifone detector is omitted.

In this arrangement, as in that of Fig. 2, the principal effect of thefeedback is to remove the distortion and noise originating in the lowfrequency signal amplifier.

Fig. 7 illustrates a reversed feedback system in which the wave fed backis a component of the plate current in a class B power amplifier. Thecarrier source I2 is connected to the antenna 2 through a carrier supplyamplifier 30, modulating amplifier l3 and power amplifier I1. Thecarrier amplifier 30 is provided with a choke coil 3I which is common tothe plate and grid circuits of the amplifier. The signal source 4 iscoupled to the grid of the tube I4 through the transformer 5 and thebridge 6. The plate circuit of the tube I4 is coupled to the gridcircuit of the tube I3 by means of a resistance 32. A choke coil I8 inthe plate current return path of tube I! is coupled to the grid of tubeI4 through the bridge 6. All the amplifiers may be class B operated.

In the operation of the system of Fig. '7, the carrier input to themodulator is substantially freed from noise effects by negative feedbackin the supply amplifier 30. The vacuum tube being operated as a class Bamplifier, any variation in the carrier amplitude results in afluctuation of the average plate current. The fluctuation is transmittedin reversed phase to the grid circuit by coil 3|, producing acompensatory variation in the gain of the amplifier. That is, anincrease of plate current causes the grid potential to become morenegative, decreasing the gain, and a decrease of plate current makes thegrid become less negative, increasing the gain. The controlled carrierwave is impressed upon the grid of tube [3 together with the signal wavefrom tube I4, the resulting modulated wave appearing in the platecircuit of tube I3. The modulated wave is amplified in tube H, theamplified wave being radiated by the antenna 2. The low frequencycomponent of the plate current of tube I1 is fed back upon the grid oftube I4 by means of choke coil I 8. The system diminishes noise anddistortion arising in amplifiers I3, I4 and I1 and also noise arising ingenerator I2 and tube 30.

What is claimed is:

l. A modulating system comprising a modulator tube and an amplifiertube, grid and plate circuits for each of said tubes, a carrier sourcecoupled to the grid circuit of the modulator, a signal source, meansresponsive to the difference between the instantaneous values of any twowaves impressed thereon, said means being connected between said signalsource and said amplifier tube, and said signal source being isolatedfrom said modulating device except through said differentiallyresponsive means, an impedance element common to the plate circuits ofboth tubes, and a detector associated by high frequency coupling withthe modulator plate circuit and by low frequency coupling with saiddifferentially responsive means.

2. A modulating system comprising a modulator tube and an amplifiertube, grid and plate circuits for each of said tubes, means formaintaining carrier waves in the plate circuit of the modulator tube, asignal source, means responsive to the difference between theinstantaneous values of any two waves impressed thereon, said meansbeing connected between said signal source and said amplifier tube, andsaid signal source being isolated from said modulating device exceptthrough said differentially responsive means,

an impedance element common to the plate cira cuits of both tubes, and adetector associated by high frequency coupling with the modulator platecircuit and by low frequency coupling with said differentiallyresponsive means.

' ENOCH B. FERRELL.

